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Ohm's Law Calculator

Enter any two of Voltage, Current, Resistance, and Power — instantly get the other two. The essential electrical formula tool.

Enter any two values — the other two are calculated instantly.

V
V

Volts — electrical pressure

I
A

Amperes — electrical flow

R
Ω

Ohms — opposition to flow

P
W

Watts — energy consumed

Enter any two values to calculate
Voltage (V)
V = I × R
V = P / I
V = √(P × R)
Current (I)
I = V / R
I = P / V
I = √(P / R)
Resistance (R)
R = V / I
R = V² / P
R = P / I²
Power (P)
P = V × I
P = V² / R
P = I² × R

About This Tool

Ohm's Law is the foundation of electrical engineering: Voltage = Current × Resistance (V = I × R). Combined with the power equation (P = V × I), it lets you calculate any electrical quantity if you know two others. This calculator handles all four variables — voltage (V), current (I), resistance (R), and power (P). Enter any two values and the other two are computed instantly. No need to remember which formula to use — the tool detects which fields you've filled and applies the correct equation automatically. Whether you're a student solving homework, a hobbyist designing a circuit, or an electrician sizing a wire — this is the one calculator you'll use every day.

How to Use

1. Enter any two of the four values: Voltage (V), Current (A), Resistance (Ω), Power (W) 2. The other two values are calculated automatically 3. Click "Reset" to clear all fields and start over 4. Use the formula reference below the calculator to understand the math 5. Click "Copy Link" to share a specific calculation

Formula

V = I × R I = V / R R = V / I P = V × I P = I² × R P = V² / R V = P / I V = √(P × R) I = P / V I = √(P / R) R = P / I² R = V² / P

Frequently Asked Questions

What is Ohm's Law?
Ohm's Law states that the voltage across a conductor is equal to the current flowing through it multiplied by its resistance: V = I × R. Voltage is measured in volts (V), current in amperes (A), and resistance in ohms (Ω). It was formulated by German physicist Georg Ohm in 1827.
How do I calculate watts from volts and amps?
Power in watts equals voltage times current: P = V × I. For example, a 120V circuit drawing 10A uses 1,200 watts (1.2 kW). This is the most commonly used power formula for household electrical work.
How do I calculate resistance from voltage and current?
Divide voltage by current: R = V / I. For example, if a 12V battery drives 0.5A through a circuit, the resistance is 12 ÷ 0.5 = 24 ohms. This is the basic rearrangement of Ohm's Law.
What is the relationship between ohms, volts, amps, and watts?
These four quantities are all interconnected. Voltage (V) is the electrical pressure, current (A) is the flow rate, resistance (Ω) opposes the flow, and power (W) is the energy consumed. Knowing any two lets you calculate the other two using Ohm's Law and the power equation.
How many watts is 120 volts?
You can't convert volts to watts without knowing the current or resistance. A 120V circuit could be 120W (at 1A), 1,200W (at 10A), or 1,800W (at 15A). Use this calculator — enter 120V and your known current or resistance to get watts.
How do I find voltage if I know watts and amps?
Divide power by current: V = P / I. For example, a 60W light bulb drawing 0.5A operates at 60 ÷ 0.5 = 120 volts.
Does Ohm's Law apply to AC circuits?
Ohm's Law applies directly to DC circuits and to purely resistive AC loads (heaters, incandescent bulbs). For AC circuits with inductors or capacitors, you must use impedance (Z) instead of resistance (R), and account for power factor. For basic household calculations with resistive loads, V = I × R works fine.

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